https://www.chewy.com/petsafe-big-d...wtJRpQXPJ5sxl7DOGrRYlxoCbEbw_wcB&gclsrc=aw.ds
BTW this is the collar I ordered upon recommendation from dog trainer at pet store I frequent. Unfortunately she couldn't offer advice on livestock aggression.
Do the zapping before dog actually launches attack. Once dog commits it takes a lot of zapping that is more likely to confuse dog than get it to back of. I would zap when dog focus on birds. Even that will frustrate dog.